1. Field of the Invention
The present invention relates to a platform, i.e., a supporting structure, containing at least one transducer for reading from and/or writing on a data carrier. It is applicable in particular to the magnetic peripherals of data processing systems, such as disc memories.
2. Description of the Prior Art
In data processing systems, the application of magnetic disc memories is becoming increasingly frequent because of their storage capacity and of the relatively short access time. Access time is the period taken by a read and/or write transducer to gain access to data contained at a particular point on the discs from the instant in which the transducers receive the order from the said processing system to gain access to this data.
Magnetic discs conventionally carry data on circular and concentric recording tracks which have a radial width not exceeding several hundredths of a millimeter, and covering the major part of their two surfaces. If a memory of this kind comprises several discs, the discs are parallel, co-axial and most frequently of the sare diameter. Whether the memory comprises several discs or one only, the disc(s) is driven at a constant speed of revolution by an electric motor.
In current practice, each surface of a disc commonly has associated with it a single transducer. To make an appreciable reduction in the time spent to gain access to any data item contained on this surface, the latter may have associated with it several transducers, of which the number does not exceed one or two tens of units and remains considerably smaller than the number of tracks. In the remainder of the description, the term "disc surface" shall imply the association of the surface with one or more transducers.
The transducers associated with one surface of a disc are generally carried by one and the same principal member installed as part of a platform which also comprises a suspension device secured to this principal member. By extension, it is said that the principal member (and equally the platform) is associated with the said surface.
There are two conventional types of platforms, namely:
(1) platforms of the movable kind which are displaced radially above the disc surface associated with them. Radial orientation of the platform enables the transducer(s) carried by their (its) principal member to be positioned so as to be able to gain access to any information contained on the associated surface. In this case, the suspension device of the platform is installed on a carriage or slide which is displaced at the outside of the discs.
(2) platforms of the stationary kind which, during the read and/or record operations, remain permanently still above defined tracks on the disc surface with which the transducers of their principal member are associated, each of these remaining associated in time with one and the same track. In this case the platform suspension device is secured to the framework of the disc memory.
It is apparent that notwithstanding its nature of the platform, (stationary or movable) the platform is secured via its suspension device to supporting means which is a carriage for a platform of the movable kind, and a framework of the disc memory for a platform of the stationary kind. The platforms most frequently utilized in current practice, are of the movable kind.
Most often, the transducers form a magnetic circuit around which is arranged a winding, and which comprises an air gap. This winding is connected by electrical input and/or output conductors to the electronic read and/or record circuits of the disc memory which comprises the transducers.
The variation of the magnetic induction within the air gap of each transducer allows of reading and/or recording (writing) data on the surface of the disc associated with it. Because of this, the air gap consequently constitutes the transducer device for picking up (reading) and/or recording (writing) data.
As a rule, but not exclusively, the principal member of a platform has the form of a comparatively flat right angle parallelepiped of which a first "large surface" positioned opposite the surface of the disc contains the air gap or gaps of the transducer(s), the second "large surface" extending substantially parallel to the first, containing the ends of the input and/or output conductors attached to the transducer(s).
During the rotation of the disc, a cushion of compressed air is formed between the disc and the first "large surface" of the principal member, which prevents this member touching and consequently damaging the disc. It is then said that the principal member flies above the disc.
The distance between the air gap(s) of the transducer(s) and the disc surface is referred to as "flight altitude" or else "flying leight" of the principal member. It is usually of the order of several tens of microns. The air cushion exerts a pressure on all or part of the first "large surface" of the principal member at right angles to the same and directed towards its second large surface". The part of the "first large surface" which is exposed to the pressure of the air cushion is referred to as "flying surface". The resulting force is referred to as "lift of the principal member" which is a function of the flying surface.
The dynamic balance of the principal member in flight is obtained by opposing to the lift an equal and opposite force referred to as "loading force" which is commonly applied to the "second large surface".
Platforms known under the name of "Winchester" platforms are utilized ever more frequently in current practice. Platforms of this nature are produced in particular by the company "Applied Magnetic Corporation" (A.M.C.) under reference numbers 3304 and 3305.
A platform of the "Winchester" type is produced in the following manner.
One or more chemicals (which are often referred to as grooves) are formed on the lower surface of its principal member, their depth being greater than or equal to 30 microns, and being able to reach up to several tenths of a millimeter. The result thereof on this surface is the presence of several projecting parts which are referred to as skids. In common practice the skids are ski-shaped.
The surface of the skids opposite the surface of the disc forms the flying surface. The lift being proportional to this surface, it is apparent that the grooves made in the lower surface of the principal member make is possible to reduce the lift and consequently the loading force by reducing the flying surface (as compared to a principal member lacking grooves). Said loading force is provided, for example, by a preloaded spring associated with the platform suspension device. (In particular cases in current practice, the said spring forms all or part of the suspension device itself).
This loading force is comparatively small and of the order of ten to twenty grammes. Because of this, it is said that the "Winchester" type of platform is a low-load platform.
As a rule, flight of a principal platform member of the "Winchester" type above the surface of the disc associated with it, occurs in such manner that the surface of the skid is generally sloping with respect to the surface of the disc. This endows said skids with sufficient lift for the flight of the principal member to be comparatively stable.
The low-load platforms of the "Winchester" type which consequently have low flight altitude, make it possible:
(1) on the one hand, thanks to this low flight altitude, to obtain read signals delivered by its (their) transducer(s) which are of relatively great amplitude, which allows of satisfactory reading reliability,
(2) on the other hand, thanks to their low load, to prevent the principal member of the platform from striking the disc and consequently damaging this latter as well as the said principal member.
However, in view of the low loading force, these platforms have certain disadvantages. For example, the probability that the flight of their principal member above the disc surface associated with it is comparatively unstable, cannot be disregarded. Further, the principal member of the suspension device is exposed to other forces than the loading force such as "parasitic forces". These parasitic forces are caused, for example, by:
(1) vibrations generated by different elements of the disc memory, such as the disc driving motor, or the motor, or the motor for driving the carriage which carries the device for suspension of the platform;
(2) inherent oscillatory displacements of the principal member around a median position which causes the same to move away from or towards the dics; and
(3) air turbulence close to the surface of the disc associated with the transducer of the principal member of the platform.
If the disc memory comprises several discs, this turbulence is particularly large in the space present between two discs. The resultant forces act essentially on the suspension device. The forces caused by the turbulence are counted among the most substantial of the parasitic forces.
It can be readily appreciated that these parasitic forces cause the flying height of the principal member to vary appreciably which causes very appreciable variations in the amplitude of the read signals, which for their part may lead to faults in determing the value of the data recorded on the discs. It is of importance, therefore, to reduce the magnitude of the said force, either by suppressing all or part of these, or by restricting their effect.
In current practice, the parasitic forces caused by inherent vibrations of the principal member of the platform are reduced by placing its suspension device as close as possible to the disc surface with which the principal member is associated.
To this end, the suspension device is secured within a groove in the principal member, the groove for example being formed in the "second large surface" containing the extremities of the input and/or output conductors of the transducers of this member. The suspension device may equally be secured within a groove formed in the "first large surface" situated opposite the surface of the disc associated with the transducer of the principal member. This last solution is described in greater detail in the French patent application Ser. No. 78 15765 filed May 26th 1978, entitled "Platform Having At Least One Transducer For Reading From And/Or Writing On A Data Carrier", and corresponding U.S. application Ser. No. 37,725, filed May 16, 1979 now U.S. Pat. No. 4,261,024, both of which are assigned to the assignee of the present invention.